Planar coils and planar transformers are used as choke coils and transformers in switching power supplies and the like. The planar coils and planar transformers have a winding made of a patterned conductor that is formed by arranging a plate-like conductor into a flat-spiral shape. In the planar transformers, or the planar coils having a plurality of windings, the windings are stacked in a direction of thickness, with an insulating layer interposed between adjacent ones of the windings.
Among the planar coils and planar transformers, the ones that deliver relatively small output currents are formed by, for example, stacking a flat-spiral-shaped patterned conductor, an insulating layer, and a magnetic layer by thin-film forming techniques such as sputtering. On the other hand, for the ones that deliver medium output currents, employed are: printed coils formed by stacking double-sided printed circuit boards with an insulating layer interposed therebetween, in which flat-spiral-shaped patterned conductors are formed on both surfaces of each printed circuit board by etching conductor layers disposed on both surfaces of the same; or coils formed by stacking flat-spiral-shaped patterned conductors with an insulating layer interposed therebetween, the patterned conductors being formed by die-cutting a conductor plate. Those coils have a hole penetrating therethrough in a direction of thickness at a center portion of the patterned conductors. A magnetic substance such as an EE-type ferrite core is inserted in the hole.
Since such planar coils and planar transformers as mentioned above can be formed to have a smaller thickness, they are used for a compact and thin switching power supply and so on, in particular.
In recent years, because of decreased operating voltages and increased currents in ICs (Integrated Circuits) resulting from an increase in their scale of integration, it has been desired that a switching power supply be reduced in size and provide a large current. A loss caused by the resistance of a conductor in choke coils or transformers, i.e., the copper loss, increases in proportion to the square of the value of the current. For this reason, it is significant to reduce the resistance value of conductors in the planar coils or planar transformers which are used as choke coils or transformers.
Switching devices such as FETs (field effect transistors), one of major components of a switching power supply, have been reduced in both loss and size as the semiconductor technology has progressed. In contrast to this, it is difficult to reduce the size of magnetic components such as choke coils and transformers, the other major components of the switching power supply. For this reason, the ratio of the volume of the magnetic components to the volume of the entire switching power supply tends to increase. Although the magnetic components are under progress toward miniaturization, this depends largely on a fact that a switching frequency has become higher due to progress in the switching devices. If a higher switching frequency is achieved, it is possible to achieve a reduction in both size and loss of the core of the coil or the transformer. This, however, present a problem that the copper loss that is a loss in the conductor increases due to the skin effect.
Conventionally, most planar coils or planar transformers have a winding in which every per-turn portion is constant in width. However, in this case, resistance becomes higher at the outer portions of the winding, which consequently causes an increase in the resistance of the entire winding.
To cope with this, Published Unexamined Japanese Patent Application (KOKAI) Heisei 5-226155 discloses a technique of increasing the width of the winding of a coil with increasing distance from the center so that every portion of the winding has the same copper loss. In this technique, the width of each portion of the winding is determined by using complicated equations. Published Unexamined Japanese Patent Application (KOKAI) Heisei 7-37728 also discloses a technique of increasing the width of the winding of a coil with increasing distance from the center so that every portion of the winding has the same or substantially the same copper loss. In both of these techniques, a ratio between Ri and W, or Ri/W, where Ri represents the radius of an inner circumference of each per-turn portion of the winding and W represents the width of each per-turn portion of the winding, is made constant, thereby allowing the copper loss to be the same for every portion of the winding. This is intended to minimize the copper loss for the entire coil in a limited space.
However, it is not proved that the copper loss for the entire coil is minimized by making the Ri/W constant.
While the number of turns of the winding (the number of winding turns) in choke coils or transformers is determined in accordance with a ripple voltage and the input/output voltage ratio required of the switching power supply, and further with the power supply driving frequency, the shape and physical properties of the core, and so on, there are many cases in which an odd number of turns are required. Printed coils allow greater flexibility in design of windings as compared with coils employing wires. For example, for printed coils, it is possible to form a winding of a desired number of turns within a specific winding frame (or an area where to place a patterned conductor) by changing the width of the patterned conductor. Furthermore, for printed coils, a plurality of patterned conductors having the same pattern may be stacked and connected in parallel to each other using a through-hole or the like, thereby allowing adjustment of permissible current capacity.
Conventionally, for planar coils or planar transformers, the following four methods have been employed for forming a winding having an odd number of turns which is equal to or greater than three. A first method is to form the winding having a required odd number of turns by using one conductor layer that includes a patterned conductor of an odd number of turns. A second method is, as shown in, e.g., Published Unexamined Japanese Patent Application (KOKAI) Heisei 4-113605, to connect an odd number of conductor layers in series to each other, each of the conductor layers including a patterned conductor of one turn. A third method is to connect a conductor layer including a patterned conductor of an even number of turns and a conductor layer including a patterned conductor of an odd number of turns in series to each other. A fourth method is, as shown in FIG. 6 to FIG. 9 of Published Unexamined Japanese Patent Application (KOKAI) Heisei 10-163039, for example, to connect a conductor layer including a patterned conductor of the [even number+α] number of turns (where α is greater than zero and less than one) and a conductor layer including a patterned conductor of the [even number+(1−α)] number of turns in series to each other.
However, the aforementioned methods have the following problems. In the first method, one of terminals of the winding needs to be drawn out from the neighborhood of an inner edge of the patterned conductor. For this reason, in the first method, it is impossible to use a core typically employed for planar coils, that is, a core in which a connecting portion that connects the portion penetrating the winding (the so-called middle foot) to the portions facing the outer circumference of the winding (the so-called outer feet) has such a great width as to cover most part of the winding. To employ the first method, it is necessary to use a core in which the aforementioned connecting portion is small in width so as not to contact with the terminal of the winding to be drawn out from the neighborhood of the inner edge of the patterned conductor. In this case, to secure a sufficient cross-sectional area of the core to avoid saturation of a magnetic flux, it is necessary to increase the thickness of the core. Thus, it is difficult for the first method to make the planar coils or planar transformers smaller in thickness.
In the second method, conductor layers as many as the number of turns required have to be stacked, which presents a problem that the planar coil or the planar transformer becomes greater in thickness. In addition, in the second method, connecting portions required for connecting an odd number of conductor layers in series to each other increase in number with increasing number of turns required. For example, forming a winding of five turns requires four connecting portions other than the terminals. This necessitates a wide area in the planar coil or the planar transformer for accommodating the connecting portions. Additionally, the second method allows a low degree of flexibility in designing the number of conductor layers because the number of conductor layers must coincide with the number of turns of the winding. For example, to form a winding of five turns, the number of conductor layers must be set in five-layer increments. In this case, for example, to increase the number of conductor layers so as to increase current capacity, the number of conductor layers can only be made equal to a multiple of five. It is therefore impossible to provide, for example, eight or twelve layers to achieve a desired current capacity.
For the third and fourth methods, the patterned conductors in the two conductor layers can be wound in directions opposite to each other to electrically connect the inner ends of the two patterned conductors to each other. This makes it possible to draw out the two terminals of the winding from the outer ends of the two patterned conductors. Thus, in the third and fourth methods, both terminals of the winding can be disposed outside the core, and this allows use of a core that is small in thickness and has a wide connecting portion between the middle foot and the outer feet. Furthermore, in the third and fourth methods, it is possible to design the number of conductor layers in two-layer increments, which allows a high degree of flexibility in designing the number of conductor layers.
Third and fourth methods, however, cause great differences between portions of the patterned conductor in width, resulting in variations of the current density from portion to portion of the winding. For this reason, the third and fourth methods cannot allow an optimum design of a patterned conductor from the viewpoint of reducing loss.